The busy backgrounds are a problem in the first two. I know how tough it is to get clean backgrounds at ALA (which is why I'm not there currently, actually), but there are a few ways of dealing with the issue. Using a longer lens from farther back means you'll pick up a much smaller piece of the background, which means you can angle the shot to find something less distracting. With the Batman shot, I would've waited for the guy in black to get out of the shot.

With a cooperative cosplayer, you can always take them a few steps away to a less distracting background, as well, or, if you want to bring gear into the equation, you could use a fast (f/2 or better) lens to totally blur out the background. An 85 f/1.8 or 50 1.4 is a great lens for doing just that.

The first image has nice poses. The lighting is pretty on their faces too. I like the snow effect, although the location does not match it. Maybe you were going for humor though. Then, I like the irony. That hotel location does make it challenging to find less distracting backgrounds, but it can be done with different angles, and lens choices.

Batman's pose is nice overall, but his eyes look toward the ground seems odd. Looking where he is aiming his fighting stance would work well. You framed him a bit frame right, which puts too much room behind him. Leading the action usually makes for a better composition. Having people walk behind him is distracting. The dark shirt and shorts of the guy with the baseball hat blends in with Batmans costume. The red wigged cosplayer's hair makes it seem like Batman is wearing a red rose in his hair. I like the vinette effect around the edges.

I don't understand Speed Racer's pose. I think you were trying to make it seem like he is running, but it seems contrived. The angle of his arm hides his upper arm making it seem like his elbow is connected to his shoulder. The badge is very prominent. It would not be too difficult to photoshop it out. The over-all color balance has a yellowish/orange cast. You did capture the fun he seemed to be having.

The top 2 are incredibly well done. I like the little bit of snow adding to the ice climbers!
The dark edges make sense in the second pic, but not the third. It shouldn't have had them in my opinion. I say choose something happier and brighter given the mood of the photo!

The top one is my fave. Very cute and I actually like the snow. Kinda makes it look like a postcard.

I forget to do this a lot, but in the last photo ask for the cosplayer to remove or swing the convention badge around to their back. It can be an awesome pose and great lighting but the viewer can be quickly distracted by a visible con badge.

Then again if you have a million convention photos in one folder then having badges visible can help you tell which convention you shot it at.